There's a whole load of health issues that can come from it. For example it probably contributed to my developing of asthma. I needed surgery, and I'm still learning how to use my nose. Hardest to fix is while sleeping.

If you are a child like anon's nephew, mouth breathing will mess up your craniofacial development.

If you start shoving food up your nose, then yes.

It's bad for your lungs because your sinuses humidifies air before it gets there. Plus it causes dry mouth and bad breath and distorts the growth of your pallette, sinuses, and face, easily leading to a deviated septum. It's also correlated with a lower IQ.

I used to be allergic to cats, and my family had cats growing up, so I was a mouth breather for most of my childhood until my body finally figured out that cat hair isn't some secret bacterial plot to destroy me.

I had a bunch of sores in my mouth from it getting too dry, and I believe my teeth suffered a bit too, but I was lucky that they were all baby teeth.

It also just took a decent amount of time for me to learn how to rely on my nose for breathing after not using it for so long. I eventually learned, but it took effort for me not to just default to the breathing method that didn't require concentration at first.

You'll get a dry mouth and bad breath. Also might need nasal polyps sorted or hayfever meds or something if your nose is constantly blocked. Also not a good look if your resting state is a slack jaw appearance.